• communication system
• earth observation
• electric propulsion
• mission analysis
• ram-ep
• scientific payload
• vleo

The advancement of space propulsion has always played a crucial role in the Earth Observation scenario. The air-breathing electric propulsion, also called ram-EP, represents a recent but widely investigated propulsion concept. The advantage of the ram-EP technology is the ability to generate thrust in Very Low Earth Orbit (VLEO) altitudes (160-300Km) without the need to store propellant on-board. Indeed, the atmospheric particles responsible for generating drag at these altitudes are used as propellant, after their ionization, to produce thrust.
The present work focuses on the mission analysis of a platform based on the ram-EP technology. In particular, the considered platform relies on SITAEL's air-breathing double-stage Hall-effect thruster (ram-HET) as propulsion system and on the heritage of GOCE for the spacecraft configuration. Unlike common mission analysis, performed within a technology-pull frame, this work was based on a technology-push approach. This choice was motivated by the fact that on one side the air-breathing technology imposes complex interactions between the propulsion subsystem and the whole spacecraft and mission design (e.g. in terms of drag, power, altitude), and on the other side the propulsion subsystem represents the enabling technology that allows performing the mission.
The final scope of the present analysis is to assess the suitability of an air-breathing platform to operate in a VLEO environment.
The first step has been to investigate the temporal evolution of space missions and their fields of application (Earth science, optical and radar imaging), considering satellites designed for Earth Observation and adopting both chemical and electrical propulsion. The analysis has been conducted for all kinds of platforms, from nanosatellites to larger spacecrafts, providing the market trends and their evolution. Particular attention throughout the entire work has been paid to the spacecrafts with a launch mass between 500 and 1500 kg, values which are close to the RAM-EP target scenario. Successively, after identification of the RAM-EP platform requirements, an overview of the VLEO environment has been performed. This investigation allowed to assess the advantages and challenges brought both to the spacecraft and the scientific payload by these orbits.
The first part of the work allowed us to identify three critical issues of VLEO missions, which are related to the mass and power allocations on board the spacecraft and to the communication system.
The first two criticalities have been investigated from the perspective of a scientific payload. Indeed, after having identified a set of feasible instruments to operate in the VLEO scenario, a preliminary scaling has been performed. The outcome of this process provided a base to perform the comparison between the scaled instruments and those adopted on chemical and electrical propulsion satellites.
The third critical issue has been tackled with the aid of a Multi-Disciplinary Optimization process. In the field of engineering, MDO is a commonly employed numerical tool for the optimization of a system. The different design features (thrust, drag, satellite’s shape and subsystems design) interact with each other and are optimized to provide a globally optimal design that satisfies the imposed constraints. For this purpose, the features related to an optical imaging camera and the communication architecture have been modelled in a numerical code, already under development at SITAEL, for the ram-EP optimization. In particular, the additional features have been specifically designed for a hypothetical imaging mission with a panchromatic and multispectral camera.
The instrument optimization provided a complete characterization of the performances concerning both image properties and the data-rate generated. This last parameter, together with a design of the instrument memory, served as input for the evaluation of the communication link budget with the Eb/N0 method. In particular, the obtained results allowed the characterization and the design of the antenna, which has then undergone a preliminary validation by comparison with off-the-shelf available space antennas. The optimization took into account also the capability of embedding the antenna on the edge of the spacecraft’s wings, considering the impact of this solution on the drag and on the available surface for the solar arrays.
From the analysis has emerged the capability of the ram-EP technology to compete, in the framework of Earth Observation missions, in terms of scientific payload’s performances, reduction of physical parameters like mass and volume as well as the capability to open new promising mission scenarios. The design of the communication system, instead, has shown that, if a compromise between downlink data-rate and transmitting power is achieved, issues regarding the shorter communication window in VLEO can be overcome. Moreover, the designed antenna presented good performances and the lack of integrational problems.